Tires are sometimes provided with one or more components of a rubber composition comprised of at least one conjugated diene-based elastomer which includes a styrene/butadiene elastomer for which the rubber composition contains reinforcing filler comprised of precipitated silica.
It is desired to evaluate the use of a specialized styrene/butadiene elastomer having a molecular weight profile restricted to having a low molecular weight content limited to a maximum of six percent of weight average molecular weight (Mw) of up to 150,000.
While a mechanism might not be fully understood, it is theorized that the presence of a low molecular weight content in a styrene/butadiene elastomer can promote absorbing greater energy within the elastomer. Such energy absorption may thereby contribute to promoting a higher hysteresis of the cured rubber composition, as may be evidenced by one or more of its rebound and tangent delta physical properties, and thereby promote higher heat generation within the rubber composition, or tire component, during tire service. It is envisioned that a lower content of low molecular weight portion of the elastomer might beneficially promote less internal heat generation within the rubber composition during its dynamic use and an associated beneficial reduction in the rubber composition's hysteresis.
Therefore, such evaluation is to be undertaken.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing elastomers blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art. The molecular weight of an elastomer may be determined, for example, by Gel Permeation Chromatography as would be understood as being well known by those having skill in such art.
Disclosure and Practice of the Invention
In accordance with this invention, a pneumatic rubber tire is provided having a component of a rubber composition containing, based on parts by weight per 100 parts by weight of the rubber (phr),
(A) conjugated diene-based elastomers comprised of:                (1) about 40 to about 90, alternately about 60 to about 80 phr of a specialized styrene/butadiene elastomer having a molecular weight profile comprised of a weight average molecular weight (Mw) in a range of from about 500,000 to about 800,000 with a low molecular weight content limited to a maximum of six, alternately about 5.5, percent of said styrene/butadiene elastomer having a number average molecular weight (Mw) of up to 150,000, and        (2) about 60 to about 10, alternately about 40 to about 20, phr of at least one additional conjugated diene-based elastomer,        
(B) about 40 to about 150, alternately about 40 to about 120, phr of reinforcing filler comprised of a combination of rubber reinforcing carbon black and precipitated silica (e.g. containing from about 0.1 to about 100 weight percent of said precipitated silica),
wherein said precipitated silica is provided together with a coupling agent for said precipitated silica having a moiety reactive or interactive with hydroxyl groups (e.g. silanol groups) on said precipitated silica and another different moiety reactive or interactive with said diene-based elastomers.
In one embodiment, said coupling agent is comprised of an alkoxyorganomercaptosilane or an bis(3-triethoxysilylpropyl) polysulfide having an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge.
The bis(3-triethioxysilylpropyl) polysulfide may have an average of from about 2 to about 2.6 or about 3.4 to about 3.8, connecting sulfur atoms in its polysulfidic bridge.
In one embodiment, said precipitated silica and coupling agent are provided as a composite of said precipitated silica pre-treated (pre-hydrophobated) with said coupling agent prior to addition thereof to the rubber composition. In such embodiment, said coupling agent may desirably be, for example, said alkoxyorganomercaptosilane.
In one embodiment, said precipitated silica and coupling agent are provided with said rubber composition to thereby provide in situ treatment of (reaction with) said precipitated silica within said rubber composition (instead of being pre-reacted to form a composite before addition to the rubber composition). In such embodiment for such in situ treated precipitated silica, said coupling agent may desirably be, for example, said bis (3-triethoxysilylpropyl) polysulfide.
In further accordance with this invention said rubber composition is provided as a sulfur cured rubber composition.
In one embodiment, the said specialized styrene/butadiene elastomer has a heterogeneity index (Mw/Mn) in a range of from about 1.5/1 to about 1.8/1.
The molecular weight characterization of the specialized styrene/butadiene elastomer (ASTM D4001-13) may be carried out using an Agilent 1260 GPC (gel permeation chromatography) system equipped with an Agilent 1260 VWD ultraviolet detector, and Wyatt Technology Light Scattering, Viscosity, and Refractive index detectors with tetrahydrofuran as the mobile phase.
In practice, such tire component may be, for example, and not intended to be limiting, a tire tread. The tread component of the tire is a circumferential tire tread intended to be capable of being ground-contacting.
Therefore, in further accordance with this invention, a tire is provided having a circumferential tread comprised of said rubber composition.
In practice, suitable rubber reinforcing carbon blacks may be found, for example, in The Vanderbilt Rubber Handbook, (1990), 13th edition, Pages 416 through 419.
In one embodiment, said additional conjugated diene-based elastomers may be, for example, elastomers comprised of at least one of isoprene and 1,3-butadiene and of styrene and at least one of isoprene and 1,3-butadiene.
Representative of such additional conjugated diene-based elastomers may be, for example and not intended to be limiting, at least one of cis 1,4-polybutadiene, cis 1,4-polyisoprene and styrene/butadiene elastomers other than, or in addition to, said specialized styrene/butadiene elastomer having said limited molecular distribution (molecular weight profile). Said additional elastomers, including said specialized styrene/butadiene elastomer, are exclusive of elastomers containing functional groups reactive with hydroxyl groups on said precipitated silica.
The aforesaid specialized or additional styrene/butadiene elastomer may, if desired, be tin or silicon coupled, more desirably tin coupled, to form a star-branched elastomer whereby its molecular weight is significantly increased.
In practice, specialized styrene/butadiene elastomer with molecular weight profile comprised of the restricted low molecular weight content has a bound styrene content in a range of from about 10 to about 40 percent. It may have a vinyl 1,2-content in a range of from about 10 to about 60 percent.
For tin coupling of any of the styrene/butadiene elastomers, various organo tin compounds, may be used. Representative of such compounds are, for example, alkyl tin trichloride, dialkyl tin dichloride, yielding variants of a tin coupled styrene/butadiene copolymer elastomer.
The precipitated silica reinforcing filler is a synthetic amorphous silica such as, for example, a precipitated silica obtained by the acidification of a soluble silica silicate (e.g., sodium silicate or a co-precipitation of a silicate and an aluminate). Such precipitated silicas are, in general, well known to those having skill in such art. Representative of precipitated silica for use in this invention includes, for example, and not intended to be limitative, Zeosil 1165MP and 200MP from Solvay, Hi-Sil 135G-D and 233 from PPG Industries, EZ160G-D from PPG Industries, Zeopol 8755LS from Huber, Ultrasil VN 2 GR from Evonik. Representative of pre-treated (pre-hydrophobated) precipitated silica is, for example and not intended to be limitative, Agilon 400™ from PPG Industries.
It is readily understood by those having skill in the art that the rubber compositions of the tread would be compounded with conventional compounding ingredients including the aforesaid reinforcing fillers such as carbon black and precipitated silica, as hereinbefore described, as well as antidegradant(s), processing oils, fatty acid comprised of, for example, stearic, oleic and palmitic acids and zinc oxide.
Processing aids may be used, for example, waxes such as microcrystalline and paraffinic waxes, in a range, for example, of about 1 to 5 phr or about 1 to about 3 phr; and resins, usually as tackifiers, such as, for example, synthetic hydrocarbon and natural resins in a range of, for example, about 1 to 5 phr or about 1 to about 3 phr. A curative might be classified as sulfur together with one or more sulfur cure accelerator(s). In a sulfur and accelerator(s) curative, the amount of sulfur used may be, for example, from about 0.5 to about 5 phr, more usually in a range of about 0.5 to about 3 phr; and the accelerator(s), often of the sulfenamide type, is (are) used in a range of about 0.5 to about 5 phr, often in a range of about 1 to about 2 phr. The ingredients, including the elastomers but exclusive of sulfur and accelerator curatives, are normally first mixed together in a series of at least two sequential mixing stages, although sometimes one mixing stage might be used, to a temperature in a range of, for example, about 145° C. to about 185° C., and such mixing stages are typically referred to as non-productive mixing stages. Thereafter, the sulfur and accelerators, and possibly one or more retarders and possibly one or more antidegradants, are mixed therewith to a temperature of, for example, about 90° C. to about 120° C. and is typically referred as a productive mix stage. Such mixing procedure is well known to those having skill in such art.
After mixing, the compounded rubber can be fabricated such as, for example, by extrusion through a suitable die to form a tire tread. The tire tread is then typically built onto a sulfur curable tire carcass and the assembly thereof cured in a suitable mold under conditions of elevated temperature and pressure by methods well-known to those having skill in such art.
The invention may be better understood by reference to the following example in which the parts and percentages are by weight unless otherwise indicated.